Single-phase synchronous motors, because of their dimensions, their efficiency and because they do not require interference suppression are suitable as a drive motors for apparatus with a rotary working motion. Such apparatus, for example citrus-juice extractors (EP-OS No. 103,930 - PHD No. 82-103 EP). Single-phase synchronous motors are mainly used in those cases where the driven load performs its operating function regardless of the driving direction. This is also the case in apparatus with an oscillating working motion.
In small domestic appliances driven by means of single-phase synchronous motors, the stability of rotation is found to depend to an extent on the direction of rotation of the motor. Strong alternating torques, which impair a stable operation, are superimposed on the average torque. Depending on the starting conditions of a single-phase synchronous motor, it is possible to distinguish between a positive and a negative direction of rotation. The positive direction of rotation is that direction in which the torque exerted on the rotor by the current becomes initially zero and in which subsequently, shifted by a small (positive) angle, the magnetic detent torque, which exists when the stator coils are not energized, also becomes zero, resulting in a stable rest position of the rotor.
In synchronous motor drives, up to now, the rotational stability is generally found to be substantially better for this positive direction of rotation than for the negative direction of rotation. Unfortunately, it appears that statistically single-phase synchronous motors tend to have a preferred starting direction corresponding to the negative direction of rotation.
A further drawback of single-phase synchronous motors is that such a motor is susceptible to starting problems in the case of a frictional load. If the friction torque is larger than the critical detent torque, the rotor may stall, in particular, in the position in which the main field directions of the rotor field and the stator field are parallel.
The critical detent torque is the magnetic torque which is exerted on the rotor when the stator coils are not energized and the rotor is rotated out of its rest position into the position in which the rotor-field and stator-field directions are parallel.
In dry-shavers having rotary cutters, the frictional load may increase considerably as a result of soiling of the shaving heads, which may impair starting. It is known to provide an additional starting aid for a single-phase synchronous motor. This ensures that, for the prevailing friction values, the main-field directions of the rotor field and the stator field are not parallel in the rest position and that starting is possible in this rotor position, which is offset from the parallel position. This offset from the parallel position can be accomplished by means of a cam-and-roller system in which a spring-loaded roller acts on a cam which is connected to the rotor and which is mirror-inverted about its central axis, thereby rotating the cam into a rest position which does not coincide with the parallel position (DE-OS No. 34 04 297 - PHD No. 84-022). Such a starting mechanism is also described in ETZ, Volume 30 (1978), no. 2, pages 56 to 60. From FIG. 2b therein it appears that in the case of a starting aid the overall pulsation torque produced by the spring torque and the detent torque is larger than their individual values. This does not provide for a reduction of speed fluctuations during operation. Moreover, this is not of interest in an apparatus with an oscillating working motion. However, there may be cases in which this is important, for example, when the time-dependence of the oscillating motion is critical.
Even during its synchronous operation, a single-phase synchronous motor exhibits substantial fluctuations in angular velocity of the rotor. The fluctuation amplitude may be up to approximately 40% of the synchronous value. This may be undesirable in certain domestic appliances although, in general, they need not comply with stringent requirements with respect to the stable operation. For example, in dry-shavers with rotary cutters, if the speed becomes smaller than a specific minimum speed, there may be an adverse effect - - - - - on the chance that a beard hair is caught and subsequently severed. Further, such speed fluctuations may give rise to annoying noises. The latter may also occur in shavers with an oscillating cutter motion.
In view of this, DE-PS No 14 88 267 proposes a single-phase synchronous motor with an artificially suppressed detent torque. This motor uses an additional magnet, which is rotatably mounted on the shaft, with an associated flux-return iron, whose artificial detent torque compensates for the fluctuation of the current torque in a given operating condition and in one direction of rotation only for providing a substantially constant torque.
This leads to a very intricate rotor construction with a rotor comprising at least two magnet elements which are 90.degree. shifted relative to each other, reducing the useful flux by a factor of .sqroot.2. This reduces the motor power. Moreover, a substantial magnet volume, which must be set into rotation by the motor, is needed in order to produce a compensation torque of the correct phase. In general, this strongly affects the starting performance of the motor, on account of the substantially higher overall mass moment of inertia. In addition, the entire construction becomes more bulky, more intricate and less efficient as a result of the addition of the compensation means, which in fact constitutes a second motor configuration without coils.
Until now speed fluctuations and high friction loads, upon starting, have prohibited the use of single-phase synchronous motors in dry-shavers with rotary cutters on account of the above-mentioned problems with respect to the starting performance, the shaving performance and the noise production.
It is an object of the invention to construct a device of the type defined in the opening paragraph, preferably with a rotary working motion, in such a way that in a simple, compact and efficient manner, the stability of the single-phase synchronous motor is improved to such an extent that fluctuations in angular velocity of the motor are reduced considerably; it is another object to improve the starting performance.